Implantable medical devices for producing a therapeutic result in a patient are well known. Examples of such implantable medical devices include implantable drug infusion pumps, implantable neurostimulators, implantable cardioverters, implantable cardiac pacemakers, implantable defibrillators and cochlear implants. Of course, it is recognized that other implantable medical devices are envisioned which utilize energy delivered or transferred from an external device.
A common element in all of these implantable medical devices is the need for electrical power in the implanted medical device. The implanted medical device requires electrical power to perform its therapeutic function whether it be driving an electrical infusion pump, providing an electrical neurostimulation pulse or providing an electrical cardiac stimulation pulse. This electrical power is derived from a power source.
Typically, a power source for an implantable medical device can take one of two forms. The first form utilizes an external power source that transcutaneously delivers energy via wires or radio frequency energy. Having electrical wires which perforate the skin is disadvantageous due, in part, to the risk of infection. Further, continuously coupling patients to an external power for therapy is, at least, a large inconvenience. The second form utilizes single cell batteries as the source of energy of the implantable medical device. This can be effective for low power applications, such as pacing devices. However, such single cell batteries usually do not supply the lasting power required to perform new therapies in newer implantable medical devices. In some cases, such as an implantable artificial heart, a single cell battery might last the patient only a few hours. In other, less extreme cases, a single cell unit might expel all or nearly all of its energy in less than a year. This is not desirable due to the need to explant and re-implant the implantable medical device or a portion of the device. One solution is for electrical power to be transcutaneously transferred through the use of inductive coupling. Such electrical power or energy can optionally be stored in a rechargeable battery. In this form, an internal power source, such as a battery, can be used for direct electrical power to the implanted medical device. When the battery has expended, or nearly expended, its capacity, the battery can be recharged transcutaneously, via inductive coupling from an external power source temporarily positioned on the surface of the skin.
Several systems and methods have been used for transcutaneously inductively recharging a rechargeable used in an implantable medical device.
PCT Patent Application No. WO 01/83029 A1, Torgerson et al, Battery Recharge Management For an Implantable Medical Device, (Medtronic, Inc.) discloses an implantable medical device having an implantable power source such as a rechargeable lithium ion battery. The implantable medical device includes a recharge module that regulates the recharging process of the implantable power source using closed-loop feedback control. The recharging module includes a recharge regulator, a recharge measurement device monitoring at least one recharge parameter, and a recharge regulation control unit for regulating the recharge energy delivered to the power source in response to the recharge measurement device. The recharge module adjusts the energy provided to the power source to ensure that the power source is being recharged under safe levels.
Transcutaneous energy transfer through the use of inductive coupling involves the placement of two coils positioned in close proximity to each other on opposite sides of the cutaneous boundary. The internal coil, or secondary coil, is part of or otherwise electrically associated with the implanted medical device. The external coil, or primary coil, is associated with the external power source or external charger, or recharger. The primary coil is driven with an alternating current. A current is induced in the secondary coil through inductive coupling. This current can then be used to power the implanted medical device or to charge, or recharge, an internal power source, or a combination of the two.
U.S. Pat. No. 5,713,939, Nedungadi et al, Data Communication System For Control of Transcutaneous Energy Transmission To an Implantable Medical Device, discloses a data communication system for control of transcutaneous energy transmission to an implantable medical device. The implantable medical device has rechargeable batteries and a single coil that is employed both for energy transmission and data telemetry. Control circuitry in the implantable device senses battery voltage and current through the battery, encodes those values by the use of multiplexer, and transmits the sensed and encoded values through the coil to an external energy transmission device. The external device includes a coil that is electromagnetically coupled to the coil in the implantable device for receiving the encoded signals and for transmitting energy to the implantable device. The external device decodes the transmitted values and transmits those to a controller for controlling energy transmission.
U.S. Pat. No. 6,212,431, Hahn et al, Power Transfer Circuit For Implanted Devices, discloses an external power transfer circuit which couples ac power having a fixed frequency into an implantable electrical circuit, e.g., an implantable tissue stimulator, while automatically maintaining optimum power transfer conditions. Optimum power transfer conditions exist when there is an impedance match between the external and implanted circuits. The external transfer circuit includes a directional coupler and an impedance matching circuit. The directional coupler senses the forward power being transferred to the implant device, as well as the reverse power being reflected form the implant device (as a result of an impedance mismatch). The impedance matching circuit includes at least one variable element controlled by a control signal. The sensed reverse power is used as a feedback signal to automatically adjust the variable element in the impedance matching circuit, and hence the output impedance of the external power transfer circuit, so that it matches the input impedance of the implant device, despite variations that occur in the input impedance of the implant device due to variations in implant distance and implant load.
For implanted medical devices, the efficiency at which energy is transcutaneously transferred is crucial. First, the inductive coupling, while inductively inducing a current in the secondary coil, also has a tendency to heat surrounding components and tissue. The amount of heating of surrounding tissue, if excessive, can be deleterious. Since heating of surrounding tissue is limited, so also is the amount of energy transfer which can be accomplished per unit time. The higher the efficiency of energy transfer, the more energy can be transferred while at the same time limiting the heating of surrounding components and tissue. Second, it is desirable to limit the amount of time required to achieve a desired charge, or recharge, of an internal power source. While charging, or recharging, is occurring the patient necessarily has an external encumbrance attached to their body. This attachment may impair the patient's mobility and limit the patient's comfort. The higher the efficiency of the energy transfer system, the faster the desired charging, or recharging, can be accomplished limiting the inconvenience to the patient. Third, amount of charging, or recharging, can be limited by the amount of time required for charging, or recharging. Since the patient is typically inconvenienced during such charging, or recharging, there is a practical limit on the amount of time during which charging, or recharging, should occur. Hence, the size of the internal power source can be effectively limited by the amount of energy which can be transferred within the amount of charging time. The higher the efficiency of the energy transfer system, the greater amount of energy which can be transferred and, hence, the greater the practical size of the internal power source. This allows the use of implantable medical devices having higher power use requirements and providing greater therapeutic advantage to the patient and/or extends the time between charging effectively increasing patient comfort.
Alignment of an external primary coil with the internal secondary coil is important in achieving efficiency in transcutaneous energy transfer. However, it is not always easy for the user to know when the primary and secondary coils are properly aligned. Often the user must resort to tactile information gleaned from the physical package into which the primary coil is located and a subtle protrusion under the skin approximately where the implantable medical device has been implanted. However, even perfectly aligning the physical package containing the primary coil with the protrusion of the implanted medical device may not result in optimum alignment of the primary and secondary coils. Often the primary coil or the secondary coil, or both, is not centered in the physical packages within which they are contained. Thus, even perfect alignment of the packages may result in actual misalignment of the primary and secondary coils.